If you’re dealing with the aftermath of a flood or fire, one issue you’re likely to encounter is how to evaluate the condition of electrical wiring hidden within the walls. You may hear that the only real way to examine the wiring is to open the walls so that the wiring can be visually examined. Usually however, opening every wall is a significant and costly over-reaction in most cases.
Electricians have been testing insulation on electrical wiring for over 100 years by using an insulation tester. These testing devices are commonly called a “Megger” (from MEGohm metER). The name “Megger” was trademarked in England in 1903 by the company that developed the first tester Evershed & Vignoles Limited. In the United States, the testers were sold by James G. Biddle Company, and commonly called Biddle Meggers here. Today, many companies just offer these as insulation testers.
The insulation tester functions by placing a higher than standard (but not harmful) voltage into an electrical cable and then measuring any current flow between the wires. Damaged insulation causes more electrical current flow which equates to reduced resistance. Most Meggers used in residential service generate 500 or 1,000 volts, although in industrial services units can generate up to 15,000 volts. The meter shows resistance in megohms (millions of ohms). The higher the resistance, the better the insulation between the wires of the cable or circuit. Higher is better. Manufacturers of the cables or systems will set minimum acceptable insulation levels, but in most cases, a minimum resistance of 25 to 100 megohms is considered acceptable for wiring.
Testing the wiring in a typical residence or small commercial building can usually be accomplished by a knowledgeable electrician using an insulation tester in a few hours. The areas that are accessible for testing include the fuse box or circuit breaker panel box as well as the outlets, switches, and light fixtures. The testing is usually done at the fuse box or circuit breaker panel. For safety considerations, the main fuse or circuit breaker is opened to disconnect power from all wiring in the building. A volt meter then is used to verify that all electrical power has been disconnected from the wiring. The electrician then disconnects all devices connected to the circuits. The wiring then can be disconnected from each fuse or circuit breaker. The electrician applies the tester to each set of wires and records the resistance reading.
If the resistance reading is above the minimum required (based on the 25 to 100 megohm minimum or the experience of the electrician) then the insulation on that wire or cable in that circuit is considered acceptable and the next circuit then is tested. Most residences have a single fuse or breaker panel with up to 42 circuits. In larger buildings, there may be additional panels with more circuits.
If the resistance of all circuits is above the minimum, then all of the wiring is considered acceptable. But if the wiring in one or more circuits does not meet the minimum resistance level, then further testing may be required. In most buildings, the wiring of each circuit has splices or additional connections at wall outlets or light switches. Where the circuit fails the test, individual portions of that circuit may be isolated and tested to attempt to determine if just one section of wire or cable is defective and must be replaced.
This relatively quick and simple testing by a licensed electrician or engineer can determine if there has been damage to electrical wiring and it must be replaced or if it can be reused without repairs, leaving all drywall intact in the meantime.